Changes in arousal state from waking to sleep are accompanied by dramatic changes in the electroencephalogram (EEG). The low amplitude, high frequency pattern of the awake EEG becomes dominated by high amplitude, low frequency synchronized activity in slow-wave sleep (SWS), followed sequentially by rapid eye movement (REM) sleep (Steriade et al., Science, 262:679-685 (1993). Acetylcholine (ACh) plays a key role in the transition of the different phases of sleep (Shiromani et al., Ann. Rev. Pharmacol, Toxicol., 27:137-156 (1987). SWS requires low ACh levels whereas REM sleep is characterized by high ACh content. Also, these phases of sleep have been shown to be differentially sensitive to a number of endogenous neuropeptides and cytokines, including somatostatin, which is known to increase REM sleep without significantly affecting other phases (Borbely et al., Physiol. Rev., 69:605-670 (1989).
The present invention describes the cloning and characterization of cortistatin, a novel neuropeptide that has been discovered to be a sleep-modulating molecule with effects opposing those mediated by somatostatin. Cortistatin, however, exhibits strong structural similarity to somatostatin. Thus obtaining a cDNA clone from screening brain-specific libraries, the mRNA of which clone is translated into a naturally occurring physiologically active protein, is yet a further example of such molecules described in U.S. Pat. Nos. 4,900,811 and 5,242,798.
Although cortistatin has now been determined to be the product of a different gene, because of its structural similarity to somatostatin as well as functional aspects described herein, cortistatin is a new member of the somatostatin family whose distribution is primarily restricted to GABAergic cortical interneurons.
GABAergic neurons have been shown to finely modulate the output of principal neurons of the cerebral cortex and hippocampus (Buhl et al., Nature, 368:823-828 (1994), areas that have been implicated in arousal state and complex cognitive functions, including learning and memory (Wilson et al., Science, 265:676-679 (1994).
The neuropeptide somatostatin was first described as a hypothalamic peptide that inhibited growth hormone release (Brazeau et al., Science, 179:77-79 (1973), and has since been implicated in many physiological phenomena, including hippocampal function and REM sleep generation (Danguir, Brain Res., 367:26-30 (1986). In the hippocampus, somatostatin is present largely in a particular set of interneurons. See, Hendry et al., Proc. Natl. Acad. Sci., USA, 81:6526-6530 (1984); Schemchel et al., Neurosci. Lett., 47:227-232 (1984); and Morrison et al., Brain Res., 262:344-351 (1983). Somatostatin may modulate the output of pyramidal neurons primarily by depressing neuronal excitability, in part via enhancement of the voltage-dependent potassium M current. See, Moore et al., Science, 239:278-280 (1988) and Schweitzer et al., Nature, 346:464-466 (1990). Pharmacological studies have shown that somatostatin also interacts with cholinergic (Araujo et al., J. Neurochem., 55:1546-1555 (1990) and Mancillas et al., Proc. Natl. Acad. Sci., USA, 83:7518-7521 (1986) and GABAergic (Freund et al., Nature, 336:170-173 (1988) systems, among others, thus modulating systems thought to underlie different aspects of behavior.
As shown in the present invention, despite the physical similarities between somatostatin and cortistatin, administration of cortistatin in vivo depresses neuronal electrical activity but, unlike somatostatin, induces low frequency waves in the cerebral cortex and antagonizes the effects of acetylcholine on hippocampal and cortical measures of excitability, thus providing a mechanism for cortical synchronization related to sleep.